The present invention relates in general to supporting platforms for sheet stacks for reproduction apparatus, and more particularly to a control for the sheet stack supporting platform of a reproduction apparatus which accommodates for variations in sheet evenness (flatness) to maintain the topmost sheet in a sheet stack in proper relation with a sheet feed head assembly.
In typical reproduction apparatus such as copiers or printers, for example, information is reproduced on individual cut sheets of receiver material such as plain bond paper or transparencies. Receiver sheets, of the various types, are stored in stacks and respectively fed seriatim from such stacks when copies are to be reproduced thereon. The sheet feeder for the reproduction apparatus must be able to handle a wide range of sheet types and sizes reliably and without damage. Sheets must be accurately fed individually from the sheet stack; that is, without misfeeds or multi-feeds.
A recently described highly efficient and reliable sheet feeder is shown in U.S. Pat. No. 5,344,133, issued Sep. 6, 1994, in the name of Jantsch et al. In such apparatus, a stack of sheets is stored in a supply hopper. A sheet feed head assembly, including a plenum, a vacuum source in flow communication with the plenum, and a mechanism, such as a feed belt associated with the plenum, urges a sheet acquired by vacuum in a sheet feeding direction away from the sheet supply stack. The sheet supply stack is supported so as to maintain the topmost sheet in such stack at a predetermined level in spaced relation with respect to the urging mechanism of the sheet feed head assembly. A first positive air supply directs a flow of air at the sheet supply stack to levitate the top several sheets in the supply stack to an elevation enabling the topmost sheet to be acquired by vacuum from the sheet feed head assembly plenum; and a second positive air supply directs a flow of air at an acquired sheet to assure separation of any additional sheets adhering to such topmost sheet.
It is clear that the sheet stack must be accurately maintained in operative relation with the sheet feed head assembly to assure the desired sheet feeding, in proper timing, from the stack. In the aforementioned sheet feeder, a stack height sensor monitors the location of the topmost sheet in the stack. A drive for the stack supporting platform drive then maintains that topmost sheet within an operating window relative to the sheet feed head assembly. However, the air flow for sheet separation causes the lead edge of the sheets in the stack on the supporting platform to fluff during sheet feeding. The fluffing action causes the stack height sensor to indicate that the topmost sheet in the stack is at the proper elevation relative to the feed head assembly, even when advance of the stack supporting platform is required to maintain the top of the stack at the proper feeding location. Moreover, any unevenness (i.e., lack of flatness) in the sheets of the stack, such as sheet curl for example, has an adverse effect on the ability to properly locate the top of the stack relative to the sheet feed head assembly. This is due to the fact that the unevenness alters the actual location of the top sheet relative to the sheet feed head assembly when such sheet is first detected by the height sensor.